It is believed that the black hole at the center of our own Milky Way Galaxy, Sagittarius A, is 4 million solar masses. This is the the most massive object in our galaxy. Even so, it is dwarfed in comparison with the black hole located at the center of NGC 4889, a galaxy 308 million light-years away at the center of the Coma Cluster. This elliptical galaxy is one of the brightest and largest galaxies in the Coma Cluster, and even though it doesn’t display much activity, it contains a black hole with a mass 21 billion times that of our Sun.
The black hole at the center of NGC 4889 sounds impressive. It should also be pointed out that the diameter is 80 billion miles. That’s 15 times the diameter of Neptune’s orbit. When you look at the photo by the Hubble Space Telescope you can see NGC 4889 as the largest and brightest object.
Despite the impressive numbers, the black hole is relatively inactive in that it appears to have stopped feeding. There is no burst of energy from the center and astronomers have even been able to make out new stars forming around it.
Black holes can typically be very active if gas, dust, and other debris accumulate around its accretion disc. The material is soon gobbled up, accelerated, pulled, and then stretched by the tremendous gravitational pull. This heats up the material to millions of degrees. During the active period of NGC 4889, it would have emitted up to a thousand times the energy emitted by the Milky Way, ejecting material from the disc and heating up the galaxy. However, this black hole is now dormant.
NGC 4889 did go through a “quasar phase” when the supermassive black hole was feeding. During this time it emitted powerful jets beyond the galaxy and into the Coma Cluster. Astronomers can see evidence of this by the X-ray emission extending to millions of light-years.